Field of the Invention
The invention relates to a method for monitoring the dynamic operability of a lambda sensor, disposed in the exhaust gas stream of an internal combustion engine downstream of a catalytic converter.
Pollutant emissions from an internal combustion engine can be reduced by catalytic post-treatment, using a so-called three-way catalytic converter. Efficient post-treatment, however, requires a stoichiometric mixture. To that end, mixture regulation is employed, with a lambda sensor (regulating sensor) disposed upstream of the catalytic converter in the exhaust gas tract, which periodically regulates the mixture composition within narrow limits around the desired value. To that end, lambda sensors that output a high voltage when the fuel/air mixture is rich and a low voltage when the fuel/air mixture is lean are for instance used; between these voltage values, there is a voltage jump, which is characteristic for xcex=1 (jump sensor).
The catalytic converter may become defective over the course of its operation, for instance from the occurrence of misfiring or from using leaded fuel by mistake. If the catalytic converter is defective, the exhaust gases are no longer detoxified, or only inadequately detoxified. To monitor the conversion capability of the catalytic converter, it is known, in addition to the lambda sensor upstream of the catalytic converter, also to dispose an oxygen sensor (monitor sensor) downstream of the catalytic converter, and to evaluate the output signals of the two sensors. For example, European Patent Disclosure EP 0 626 506 A1 proposes forming the ratio of the times within which the oxygen sensor downstream of the catalytic converter and the oxygen sensor upstream of the catalytic converter indicate a rich or a lean mixture value, respectively. The catalytic converter is adequately efficient if the lesser value of these ratio values is less than predetermined limit value.
The known methods for monitoring catalytic efficiency by evaluating the output signals of the two sensors assumes that the sensors themselves are functioning properly, so that no incorrect values, obtained from aged or poisoned sensors can affect catalytic diagnosis. It is therefore necessary to monitor the operability of the sensors as well. It is known from EP 0 616 119 A1, in a lambda sensor (precatalytic sensor) disposed upstream of the catalytic converter, to measure the switching times within which the precatalytic sensor, in the context of its jump function, switches over from a high voltage value (rich voltage) that indicates a rich mixture to a lower voltage value (lean voltage) that indicates a lean mixture. The magnitude of these switching times is a measure of the operability of the precatalytic sensor.
Another method for monitoring the dynamic operability of precatalytic lambda sensors is described in EP 0 652 358 A2. In it, the dwell times within which the lambda sensor signal indicates a rich or a lean mixture are measured. The precatalytic lambda sensor is then classified as functioning correctly if both the rich and the lean dwell times are below predetermined limit values associated with the individual dwell times.
These known monitoring methods for precatalytic sensors in the, closed lambda control loop cannot, however, readily be employed for diagnosis of monitor sensors, since because of the storage capacity of the catalytic converter, the course of the closed-loop control oscillation present upstream of the catalytic converter is more or less markedly smoothed, depending on the conversion capacity of the catalytic converter. Hence no closed-loop control oscillation that can be evaluated in this way is available; instead, there is only a more or less constant signal, which cannot be evaluated for monitoring the monitor sensor, or can be evaluated only with a large potential for error.
The object of the invention is to disclose a method with which the dynamic operability of a monitor sensor (jump sensor), which is disposed downstream of the catalytic converter and has a binary characteristic with regard to its output signal, can be monitored.
Whenever the engine enters the operating range of overrun fuel cutoff, the output signalxe2x80x94as a rule, an electrical voltagexe2x80x94of an intact monitor sensor drops very quickly from a relatively high value to a low value, because of the lean mixture that then prevails.
Once the engine operating state of overrun fuel cutoff is detected, the rich/lean switching time of the monitor sensor is therefore ascertained and weighted as a function of the air flow rate and the intake system. By comparison of the mean value of the switching time thus obtained with a limit value and ensuing evaluation, the operability of the monitor sensor is diagnosed. The operability is assured if the switching time is below the limit value.
By monitoring the output signal of the monitor sensor upon entering and leaving the overrun fuel cutoff phase, implausible voltage values from an extreme lengthening of the switching times, as well as breaks in the signal line of the sensor, can be detected. The advantage attainable with the invention is in particular that with the method described, it is easily possible to detect poisoning as well as effects of aging of the lambda sensor downstream of the catalytic converter, which can lead to a worsening in the exhaust gas values of the engine or to incorrect values in catalytic diagnosis. These worsened values can be ascribed to serious changes in the dynamic behavior of the monitor sensor (prolongation of the rise times from rich to lean and lean to rich, defective conversion behavior of the catalytic converter after the overrun fuel cutoff phase, or electrical errors (line breakage)
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for monitoring the operability of a lambda sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.